FIG. 1 is a block diagram of a conventional signal processing system 10. The signal processing system 10 comprises a calculating apparatus 120, a demapper 140 and a decoder 180. The calculating apparatus 120 calculates a phase error Θ according to a part of the signal, such as the pilot signal, and further generates a phase calibration value ejθ. The demapper 140 receives the signal, calibrates the signal phase according to the phase calibration value ejθ of the calculating apparatus 120 and then demaps the signal phase to generate a plurality of soft values. The decoder 180 decodes the soft values to the output data.
FIG. 2 is a block diagram of the calculating apparatus 120. The calculating apparatus 120 comprises phase calculation units 121, 122, 123 and 124, an adding unit 126, an averaging unit 128 and an exponential unit 129. The phase calculation units 121, 122, 123 and 124 receive the pilot signal P−21, P−7, P7 and P21 of the received signal respectively and calculate the individual phase of P−21, P−7, P7 and P21 respectively. The adding unit 126 adds the individual phase of P−21, P−7, P7 and P21 together and then calculates the average to obtain the phase error Θ by the averaging unit 128. Then, the exponential unit 129 generates the phase calibration value ejθ. The calculating apparatus 120 is designed according to the specification of IEEE 802.11a/g, and its detail operation is acknowledged by the people skilled in the art, so it is not repeated here.
In the signal processing system, when the signal is received by the receiving end, the signal is affected by the noise, which causes the phase error, such as sampling time offset (STO), carrier frequency offset (CFO), phase noise and sampling clock offset (SCO), etc. In the skilled art, the conventional signal processing system only considers channel frequency response in a soft-output demapper the phase errors aroused from other causes still exist in the system, and hence the signal processing system 10 cannot process the received signals correctly.
Viterbi decoder 180 is a commonly used decoder to decode the transmitted data from soft values. Viterbi decoder is able to decode the signal with phase error correctly if the information of the phase error is known by the Viterbi decoder beforehand. Therefore, it is desirable to develop a signal processing system and method to adjust the signal according to the phase error, such that the signal processing system can have better performance.